1. Technical Field
The present invention relates to a resistive memory device and a write method thereof.
2. Related Art
Recently, studies have been conducted on next-generation memory devices that can serve as substitutes for existing memory devices such as dynamic random access memory (DRAM) devices and flash memory devices. One next-generation memory device is a resistive random access memory (ReRAM) device that uses a resistive material capable of switching between two or more different resistance states by a sharp variance of resistances in response to an applied bias voltage.
The resistance of a variable resistance material layer of a ReRAM device reversibly changes according to the polarity or amplitude of a pulse applied thereto.
In addition, the ReRAM device may operate in a unipolar switching manner or a bipolar switching manner according to polarities of voltages applied in a set operation and a reset operation.
FIG. 1a and FIG. 1b are hysteresis curves showing a bipolar switching operation of a typical resistive memory device.
FIG. 1a and FIG. 1b show a positive switching manner and a negative switching manner, respectively, in a bipolar switching operation of a resistive memory device. In FIG. 1a and FIG. 1b, an X axis represents voltage, and a Y axis represents current.
In the hysteresis curve of FIG. 1a, while increasing a positive voltage, cell data is defined as reset data if it is sensed that the cell has a high resistance state and defined as set data if it is sensed that the cell has a low resistance state, with respect to a predetermined voltage. The directions of the arrows show that there are two paths for data writing.
In the hysteresis curve of FIG. 1b, while increasing a negative voltage, cell data is defined as set data if it is sensed that the cell has a high resistance state and defined as set data if it is sensed that the cell has is a low resistance state, with respect to a predetermined voltage. Similar to FIG. 1a, in FIG. 1b paths for data writing appear to have two directions.
FIG. 1a and FIG. 1b illustrate the bipolar manner in which a bidirectionally reversible reaction is possible as described above. As shown in FIGS. 1a and 1b, an operation can be performed between a set state and a reset state at a predetermined threshold voltage.
Therefore, when designing a cell structure, it is first determined whether the resistance reference of the cell is defined in a positive-reset and negative-set manner or in a negative-reset and positive-set manner. Then a type of write driver is selected according to the result of the determination.
However, characteristics of a cell may vary depending on process, voltage, and temperature (PVT) conditions of a cell manufacturing process, cell operation conditions, or ambient environments. As a result, even if all circuit conditions and schemes are designed in a positive switching manner, a specific cell may still exhibit negative switching characteristics as its threshold voltage deviates from a predetermined threshold voltage, which may cause errors in a data writing operation.